The nondestructive evaluation of materials for damage and defects often involves inspection of curved surfaces having limited access, such as engine disk slots, helicopter propulsion components, turbine blades, bolt holes, automotive components and other components with enclosed or partially enclosed regions having narrow openings. Typically, the defect is found when the inspection sensor, such as an eddy-current sensor, is brought into intimate contact with the surface. For coverage over wide areas of the surface, this inspection requires the use of sensors that are formed into the shape of the curved surface or are fabricated onto a flexible backing that can conform to the shape of the surface.
Compliant substrates, such as foam or ferrite loaded substrates, have been used to enhance the performance of eddy-current sensors and allow sensor arrays to conform to a surface through the compliance of the substrate. This is described, for example, by Goldfine (U.S. Pat. No. 5,453,689), Vernon (U.S. Pat. No. 5,278,498), Hedengren (U.S. Pat. No. 5,315,234) and Johnson (U.S. Pat. No. 5,047,719). While these non-rigid substrates offer the advantage of conforming to a wide range of complex shapes, they often require a rigid inner core to maintain the general shape. This can result in local variations in pressure on the sensor and a lack of adherence of the array to the surface of the material under test.
The shape of devices and gaps between devices has been controlled by the use of fluids such as water, air and oil for devices such as automobile tires, balloons used in angioplasty to clear arteries in the heart, and in air bearings. Often the desire is to maintain a specific shape without significant compliance after the shape has been established.